Private decisions and social allocation
| English | Français |
|---|---|
| market failure/ˈmɑːkɪt ˈfeɪlɪə/ | défaillance du marché |
| social optimum/ˈsəʊʃl ˈɒptɪməm/ | optimum social |
A decision you can investigate
- A market can clear while missing the interests of people outside the transaction. Conversely, a socially valuable service may have no paying customer.
- An equilibrium is not automatically an efficient social allocation.
Build the explanation
- Market failure · Défaillance du marché 市场失灵 occurs when the market allocates resources inefficiently: output can exceed or fall below the social optimum 社会最优. In a marginal model, efficiency compares the social benefit of one extra unit with its social cost. Private buyers and sellers may omit external effects on third parties.
- Other mechanisms are distinct: non-excludability invites free riding on a public good; information gaps distort choices; protection from losses may alter risk-taking; and speculative expectations can feed a bubble. Name the mechanism and explain its allocation consequence instead of listing every failure for every case.
Work through the evidence
- In a fictional market, marginal private benefit is MPB=100−Q and marginal private cost MPC=20+Q. Private equilibrium solves MPB=MPC: 100−Q=20+Q, giving Q=40. Suppose each extra unit also damages neighbours by 20: MSC=MPC+20=40+Q, while MSB=MPB.
- The social optimum solves MSB=MSC: 100−Q=40+Q, giving Q=30. Units 31–40 cost society more than their benefit. The market clears at 40, but this does not remove the external harm. If no external effect or other distortion existed in this model, private and social optimum would coincide.
What is private output in the stated case?
Set the private marginal benefit and cost equal.
Test the limits
- Social efficiency is not identical to fairness. A distributional argument needs explicit value judgements; a high price alone does not prove market failure. A shortage under a binding price ceiling has a different cause from omitted external costs.
- Do not claim every disliked outcome is failure or that any intervention improves it. Compare achievable alternatives, including information and enforcement costs, rather than an ideal government with an imperfect market.
What changes when neighbour damage is included?
The omitted third-party cost enters MSC.
A market-clearing price proves that social marginal benefit equals social marginal cost.
Clearing equates private demand and supply; omitted social effects can separate the optima.
Apply and explain your answer
- Why does the marginal comparison identify the last ten units as excessive?
- For Q above 30, social marginal cost exceeds social marginal benefit, so those units reduce net social gains.
Which explains a non-excludable public good’s financing problem?
Free riding can undermine voluntary payment and provision.
Use the terms precisely
- market failure: Market allocation does not achieve an efficient use of resources.
- social optimum: An allocation maximizing net social benefit within the stated model and constraints.
Match the terms to their meanings.
Use each term for its stated economic relationship.
In a fictional market, marginal private benefit is MPB=100−Q and marginal private cost MPC=20+Q. Private equilibrium solves MPB=MPC: 100−Q=20+Q, giving Q=40. Suppose each extra unit also damages neighbours by 20: MSC=MPC+20=40+Q, while MSB=MPB. The social optimum solves MSB=MSC: 100−Q=40+Q, giving Q=30. Units 31–40 cost society more than their benefit. The market clears at 40, but this does not remove the external harm. If no external effect or other distortion existed in this model, private and social optimum would coincide.
Social efficiency is not identical to fairness. A distributional argument needs explicit value judgements; a high price alone does not prove market failure. A shortage under a binding price ceiling has a different cause from omitted external costs. Do not claim every disliked outcome is failure or that any intervention improves it. Compare achievable alternatives, including information and enforcement costs, rather than an ideal government with an imperfect market.
Market failure occurs when the market allocates resources inefficiently: output can exceed or fall below the social optimum. In a marginal model, efficiency compares the social benefit of one extra unit with its social cost. Private buyers and sellers may omit external effects on third parties.